Process for forming features on a semiconductor wafer using a phase shifting mask that can be used with two different wavelengths of light

ABSTRACT

A phase shifting mask can be used with exposure lights of two different wavelengths. The depth of the phase shifting layer is calculated and fabricated such that it shifts a first exposure light about 180° and a second exposure light about 180°.

[0001] This invention was made with government support under ContractNo. MDA 972-92-C-0054 awarded by Advanced Research Projects Agency(ARPA). The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to processes for creatingphotomasks or reticles used in the fabrication of semiconductor devices.More particularly, the present invention relates to phase shiftingphotomasks and processes for designing them.

[0003] Advances in capacity in semiconductor chips have generally beenthe result of decreases in the size of the features on the chip. Thelateral dimensions of features are generally defined byphotolithographic techniques in which a detailed pattern is transferredto a photoresist by shining light through a mask or reticle.

[0004] In recent years, phase shifting masks have been developed toimprove photolithographic processes. Phase shifting masks increase imagecontrast and resolution without reducing wave length or increasingnumerical aperture. These masks also improve depth of focus and processlatitude for a given feature size.

[0005] With phase shift photolithography, the interference of light raysis used to overcome the problems of diffraction and improve theresolution and depth of optical images projected onto a target. Withthis technology, the phase of the exposure light at the target iscontrolled such that adjacent bright areas are preferably formed 180°out of phase with each other. Dark regions are thus produced between thebright areas by destructive interference even when diffraction wouldotherwise cause these areas to be lit. This technique improves totalresolution at the target.

[0006] In general, a phase shifting mask is constructed with arepetitive pattern formed of three distinct layers. An opaque layerprovides areas that allow no light transmission. A first transparentlayer provides areas which allow close to 100% of the light to passthrough. A transparent phase shifting layer provides areas which allowclose to 100% of the light to pass through but phase shifted 180 degreesfrom the light passing through the first transparent layer. The firsttransparent layer and the phase shifting layer are positioned such thatlight rays diffracted through each area are cancelled out in a darkenedarea between them. This creates a pattern of dark and bright areas whichcan be used to clearly delineate features of a pattern defined by theopaque layer on the semiconductor wafer. Another method of constructinga phase shifting mask utilizes a semitransparent layer to cause thephase shift.

[0007] One process for fabricating phase shifting masks includes formingan opaque layer on a major surface of a transparent substrate,patterning the opaque layer to expose portions of the underlyingtransparent substrate, forming a phase shifting mask layer to expose theportions of the underlying transparent substrate, phase-etching theexposed portions of the transparent substrate until a 180° phase shiftis accomplished. Other processes of fabricating phase shifting masksinclude those in which a transparent film is formed over a portion of amask to create a phase shift as well as the etching of phase shiftingchannels into the mask substrate.

[0008] Phase shifting masks are considerably more difficult to designand fabricate than conventional photomasks. Accordingly, it is desirableto fabricate phase shifting masks which will have as long a useful lifeas possible and which are capable of being used under varying exposureconditions. One of the problems associated with current phase shiftingmasks is that they must be designed for use with a specific type ofstepper.

[0009] Accordingly, it would be a significant advancement in the art toprovide a photomask and method of fabrication wherein the photomaskcould be used with different types of steppers. It would be furtheradvancement if such a photomask could be used with steppers havingdifferent wave lengths.

BRIEF SUMMARY OF THE INVENTION

[0010] The present invention provides a phase shifting photomask thatcan be used with steppers having different wave lengths of exposurelight. The present invention also provides a method for designing such aphase shifting photomask.

[0011] In a preferred embodiment of the present invention, a phaseshifting mask layout is designed and etched using conventionaltechniques. The depth of the etch for the phase shifting portion iscalculated and etched such that exposure light of two differentwavelengths will each have a phase shift of about 180°.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic cross-sectional view of a portion of a phaseshifting mask according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention provides a phase shifting photomask whichcan be used with different wave lengths of light. The invention is bestunderstood by reference to the attached drawing in connection with thefollowing description.

[0014] Referring first to FIG. 1, a portion of a phase shifting mask 10is illustrated in cross-section. Mask 10 includes a quartz substrate 12with a chrome layer 14 formed thereon. A plurality of apertures 16 and18 are formed in chrome layer 14. Apertures 16 expose the surface ofquartz substrate 12. Apertures 18 expose portions of quartz substrate 12which have been etched to a depth d to create a phase shifting layer 22.Mask 10 is designed and fabricated using tools and techniques well knownto those skilled in the art of mask design.

[0015] Mask 10 is designed such that light having a wave length ofeither λ₁ or λ₂ can be used. As the light passes through apertures 16 ithas a phase of φ₁. As the light passes through apertures 18, both wavelengths λ₁ and λ₂ have a phase of φ₂. The depth d of the etch in thequartz is such that

φ₂(λ₂)−φ₁(λ₂)=π(±2 kπ)  (1)

φ₂(λ₁)−φ₁(λ₁)=π(±2 kπ)  (2)

[0016] wherein λ equals the wave length of the light and φ equals phaseof the light and k is an integer. Mask 10 will behave like analternating aperture phase shifting mask for both λ₁ and λ₂ wavelengths.

[0017] In order to design a phase shifting mask whereby a 180° phaseshift is obtained at a first wave length and a 180° phase shift isobtained at a second wave length, it is necessary to find a common depthfor the phase shifting layer 22 which will produce the desired phaseshifts. This can be accomplished by using the formula

[0018]  d=iλ/2(n−1)  (5)

[0019]

[0020] wherein d equals depth of the etch, i equals an odd number for a180° phase shift and an even number for a 0° phase shift, λ equals thewave length of exposure light and n equals the refractive index of thematerial the light travels through. Setting d to be approximately equalfor two different wave lengths results in the following equation$\begin{matrix}{\frac{i_{1}\lambda_{1}}{2\left( {n_{1} - 1} \right)} \approx \frac{i_{2}\lambda_{2}}{2\left( {n_{2} - 1} \right)}} & (6)\end{matrix}$

[0021] since λ₁, λ₂, n₁ and n₂ are known quantities, i₂ can be expressedas a function of i₁ $\begin{matrix}{i_{2} \approx {\frac{i_{1}\lambda_{1}}{\left( {n_{1} - 1} \right)} \cdot \frac{\left( {n_{2} - 1} \right)}{\lambda_{2}}}} & (7)\end{matrix}$

[0022] This equation can be solved for various wave lengths andsubstrate materials. For example, for an i-line process with a quartzsubstrate, an etch depth of 3850 Å produces a shift of 180° such thatn₃₆₅ equals 1.474. For a deep UV process at 248 nm, a 2440 Å quartz etchshifts the light 180° resulting in n₂₄₈ equals 1. 508. Substitutingthese values into equation 5, results in

[0023]  i ₂ ≈i ₁(1.5773)  (8)

[0024]

[0025] A table can be created to help determine an appropriate value fori₁ and i₂. Using the values from equation 8 results in the followingtable: i₁ i₂  1  1.5773  3  4.7319  5  7.8865  7 11.0411  9 14.1957 1117.3503

[0026] From this table it is apparent that at i₁ equals 7, i₂ isapproximately equal to an odd integer, in this case 11.

[0027] Using a value of 7 for i₁, it is possible to calculate a depthfor the phase shifting layer which will produce a 180° phase shift forλ₁ and an approximately 180° phase shift for λ₂. Using a value of i₂equals 11 it is possible to obtain a depth which will produce a 180°phase shift for λ₂ and an approximately 180° phase shift for λ₁. Inorder to obtain a mask which works equally well at both wave lengths,the difference in the depths could be averaged. For it equals 7, dequals 26950 Å. For i₂ equals 11, d equals 26840 Å. Since a phase changeat 248 nm requires 0.634 as much depth as an equal phase change at 365nm the compromise depth can be calculated as follows:

[0028]  26950+(0.634) (26850−26950)=26887 Å  (7)

[0029]

[0030] Thus, a quartz mask having a phase shifting layer about 26890 Ådeep can be used with an i-line exposure at 365 nm and a deep UVexposure at 248 nm. It will be appreciated by those skilled in the artthat similar calculations can be performed to design a mask fabricatedfrom other substrate materials or a mask to be used with differentexposure lights. Additional substrate materials that have been used forphotomasks include borosilicate glass and soda-lime glass. Otherexposure lights include g-line at 436 nm and h-line at 405 nm.

[0031] While the invention has been described with respect to thepresently preferred embodiments, it will be appreciated by those skilledin the art that modifications and changes can be made to the presentinvention without departing from its spirit or essentialcharacteristics. For example, this process can be used with additivephase shifting masks as well as subtractive phase shifting masks.Accordingly, all modifications or changes which come within the meaningand range of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A phase shifting mask including a phase shiftinglayer shifting a first exposure light having a first wavelength about180° and shifting a second exposure light having a second wavelengthabout 180°.
 2. A phase shifting mask as defined in claim 1 wherein saidfirst exposure light has a wavelength of about 365 nm.
 3. A phaseshifting mask as defined in claim 1 wherein said second exposure lighthas a wavelength of about 248 nm.
 4. A phase shifting mask as defined inclaim 1 wherein said first exposure light has a wavelength of about 365nm and second exposure light has a wavelength of about 248 nm.
 5. Aphase shifting mask as defined in claim 1 wherein said mask comprises aquartz substrate.
 6. A phase shifting mask as defined in claim 1 whereinsaid phase shifting layer is etched into the mask surface.
 7. A phaseshifting mask as defined in claim 6 wherein said phase shifting layerhas a depth of about 26890 Å.
 8. A phase shifting mask as defined inclaim 1 wherein said phase shifting layer is added to said mask surface.9. A phase shifting mask as defined in claim 8 wherein said phaseshifting layer has a thickness of about 26890 Å.
 10. A phase shiftingmask as defined in claim 1 wherein said phase shifting mask comprises aquartz substrate, said first exposure light has a wavelength of about365 nm, said second exposure light has a wavelength of about 248 mn andsaid phase shifting layer has a depth of about 26890 Å.
 11. A phaseshifting mask formed from a quartz substrate having a surface includinga phase shifting layer shifting a first exposure light having a wavelength of about 365 nm about 180° and shifting a second exposure lighthaving a wavelength of about 248mm about 180°.
 12. A phase shifting maskas defined in claim 11 wherein said phase shifting layer is etched intosaid surface.
 13. A phase shifting mask as defined in claim 11 whereinsaid phase shifting layer is added to said surface.
 14. A phase shiftingmask used in the manufacture of semiconductors, said mask including aphase shifting layer having a depth shifting a first exposure lighthaving a first wavelength and a second exposure light having a secondwavelength approximately the same amount.
 15. A method of creating aphotomask that can be used with exposure lights having differentwavelengths comprising: patterning an opaque layer on a surface of atransparent substrate to create transmitting and nontransmittingportions; and creating a phase shifting layer shifting the phase of afirst exposure light having a first wavelength about 180° and shifting asecond exposure light having a second wavelength about 180°.
 16. Amethod of creating a phase shifting mask as defined in claim 15 whereinsaid phase shifting layer is etched into said substrate.
 17. A method ofcreating a phase shifting mask as defined in claim 16 wherein said phaseshifting layer has a depth of about 26890 Å.
 18. A method of creating aphase shifting mask as defined in claim 15 wherein said phase shiftinglayer is added to the surface of said substrate.
 19. A method ofcreating a phase shifting mask as defined in claim 18 wherein said phaseshifting layer has a thickness of about 26890 Å.
 20. A method ofcreating a phase shifting mask as defined in claim 15 wherein saidtransparent substrate comprises quartz.
 21. A method of creating a phaseshifting mask as defined in claim 15 wherein said first exposure lighthas a wavelength of about 365 nm.
 22. A method of creating a phaseshifting mask as defined in claim 15 wherein said second exposure lighthas a wavelength of about 248 nm.
 23. A method of creating a phaseshifting mask as defined in claim 15 wherein said first exposure lighthas a wavelength of about 365 nm and said second exposure light has awavelength of about 248 nm.
 24. A method of creating a phase shiftingmask as defined in claim 15 wherein said first exposure light has awavelength of about 365 nm, said second exposure light has a wavelengthof about 248 nm and said phase shifting layer has a depth of about 26890Å.